![Scatter plot of C i brightness ($I_{{\rm C} \, {\small I}}$, top) and C i/CO ratios ($I_{{\rm C} \, {\small I}}/I_{\text{CO}}$, bottom) versus CO brightness ($I_{\text{CO}}$) across different time-scales (3–8 Myr) and disc radii for AGN (left) and NoAGN (right) models. Points are colour-coded by distance from the centre of the disc. A correlation is observed between C i brightness and CO, consistent with trends in starburst regions (dotted line; Salak et al. 2019) and CNDs (dashed line; Liu et al. 2023), with AGN models exhibiting higher C i brightness. The bottom panels illustrate that simulations predict higher C i/CO ratios at fainter CO pixels, particularly at outer radii, contrasting with observations where upward tails in C i versus CO at the CO-faint end are primarily due to detection limits. The simulations accurately capture this behaviour, showing higher C i and C i/CO values for AGN models even at early times [above the dashed line representing the identity value $\log (I_{{\rm C} \, {\small I}}/I_{\text{CO}}) > 0$]. Systematic differences in C i/CO ratios are noted among galaxies, with NGC 3627 and NGC 4321 displaying a tight distribution around 0.1 (dashed line; Liu et al. 2023, presenting star-forming disc), while NGC 1808 and NGC 7469 show increased ratios in starburst regions, reaching about 0.2, indicating different galactic environments. The colour bar indicates the distance from the disc centre, and shading lines in the top panels represent the $\sigma$ levels for each part of the C i map within 200 pc.](https://oup.silverchair-cdn.com/oup/backfile/Content_public/Journal/mnras/539/2/10.1093_mnras_staf557/1/staf557fig6.jpeg?Expires=1749401221&Signature=z~eJ2cs2-cym3kH0IFYp7e7RuINcsPxWtk23yfrKxcefcbYUaCJwLr627NoDC23EHUzFMyTfJjvNO~Qj29snIG-Y41GiK3PirRwDyQubiOtZWfytLKnCw-ztCD8VaekvGSuazgafPfQpkqi4l~HuueampmlyHw90728Ng3fqOwX~xaIyf9ob8VKo-HEUZXbhshN95tuynC-HiY1WdTqcwoktKBDMtjC4Y3CygjTlVvCs0KDSiMsysH~gMchJY-ZkAubKinxS661sny2oPug2RXXIYhv8uAu6C5veMggrkxU~9yVWfhSZo50CkpU92ML1i89lfa7lflKY-BHVihAJlQ__&Key-Pair-Id=APKAIE5G5CRDK6RD3PGA)
Scatter plot of C i brightness (|$I_{{\rm C} \, {\small I}}$|, top) and C i/CO ratios (|$I_{{\rm C} \, {\small I}}/I_{\text{CO}}$|, bottom) versus CO brightness (|$I_{\text{CO}}$|) across different time-scales (3–8 Myr) and disc radii for AGN (left) and NoAGN (right) models. Points are colour-coded by distance from the centre of the disc. A correlation is observed between C i brightness and CO, consistent with trends in starburst regions (dotted line; Salak et al. 2019) and CNDs (dashed line; Liu et al. 2023), with AGN models exhibiting higher C i brightness. The bottom panels illustrate that simulations predict higher C i/CO ratios at fainter CO pixels, particularly at outer radii, contrasting with observations where upward tails in C i versus CO at the CO-faint end are primarily due to detection limits. The simulations accurately capture this behaviour, showing higher C i and C i/CO values for AGN models even at early times [above the dashed line representing the identity value |$\log (I_{{\rm C} \, {\small I}}/I_{\text{CO}}) > 0$|]. Systematic differences in C i/CO ratios are noted among galaxies, with NGC 3627 and NGC 4321 displaying a tight distribution around 0.1 (dashed line; Liu et al. 2023, presenting star-forming disc), while NGC 1808 and NGC 7469 show increased ratios in starburst regions, reaching about 0.2, indicating different galactic environments. The colour bar indicates the distance from the disc centre, and shading lines in the top panels represent the |$\sigma$| levels for each part of the C i map within 200 pc.
